1. Field of the Invention
The present invention relates to a filter element in which a strip-line resonator is disposed on a dielectric substrate. The present invention also relates to a method for manufacturing a filter element.
2. Description of the Related Art
There are previously known filter elements that have wide-band frequency characteristics utilizing a coupling of a plurality of strip-line resonators.
Japanese Unexamined Patent Application Publication No. 10-65401 discloses a filter element in which a plurality of strip-line resonators disposed on a single dielectric substrate serve as filters. The filter element includes a ground electrode on a back main surface and a side surface of the flat dielectric substrate and also includes resonant lines each having a shorting end in the vicinity of a border between the side surface and a front main surface. The shorting ends of adjacent strip-line resonators are positioned in the same orientation and the open ends thereof are positioned in the same orientation, and the strip-line resonators are comb-line coupled. The degree of strength of comb-line coupling is increased by the provision of a main-surface electrode for comb-line coupling.
Japanese Unexamined Patent Application Publication No. 7-58521 discloses a filter element in which a plurality of groups of strip-line resonators having different resonant frequencies is disposed on a single dielectric substrate. The filter element includes a ground electrode on a back main surface and a portion of a side of the flat dielectric substrate, a first group of resonant lines (strip-line resonators) extending from a ground electrode of the back main surface to the side and the front main surface, and a second group of resonant lines (strip-line resonators) extending from a wide ground electrode of the portion of the side to the front main surface. Each of the resonant lines of the first group has a shorting end in the vicinity of the border between the back main surface and the side of the dielectric substrate. Each of the resonant lines of the second group has a shorting end in the vicinity of the border between the side and the front main surface of the dielectric substrate. The strip-line resonators of the first group and those of the second group have different resonant frequencies.
Japanese Unexamined Patent Application Publication No. 10-107537 discloses a method for manufacturing an antenna element to form a surface-mount antenna utilizing a strip-line resonator. The manufacturing method described in this patent document is a method for producing an antenna element by forming a circuit pattern on a dielectric base substrate, then dividing the dielectric base substrate into antenna-element bases, and forming an electrode on a side of each of the antenna-element bases.
In a filter element or an antenna element described in the above-described documents, resonant characteristics of a strip-line resonator are determined by the shape of a line (circuit pattern) provided on a main surface of a dielectric substrate (dielectric base substrate), and components that determine the resonant characteristics are concentrated on the main surface.
As a result, it is difficult to adjust the resonant characteristics after the circuit pattern is printed on the main surface of the dielectric substrate (dielectric base substrate). If the resonant characteristics of a strip-line resonator vary from a design value in a step of printing a circuit pattern, a subsequent step of dividing into element bases, or another step, a defective element having resonant characteristics different from the design value would be produced, thus causing a problem of decreasing manufacturability of elements.